1. Field of the Invention
This invention relates to a charger for charging a battery, and more particularly it pertains to such a charger using a switching power supply device.
2. Description of the Prior Art
To charge a battery quickly without damage, it has been considered ideal to cause a constant current to flow in the battery. With constant current flow, however, there is a tendency that when the battery is sufficiently charged, a high charging voltage is applied thereto while when the battery is discharged to be substantially empty, the output voltage thereof is decreased substantially down to zero. A switching power source has been conventionally employed to charge a storage battery, but because of such a battery having the property mentioned just above, it is required that the power source be able to cope with an output voltage variation.
With this type of charger, in an attempt to keep the charging current constant, it has been the usual practice to control the pulse width of the switching signal by feeding back thereto an error signal derived from a comparator circuit for comparing output current with reference current However, when the output voltage of the storage battery goes below a predetermined level (referred to as lower limit voltage level hereinafter), the reference current is also decreased so that the comparator circuit fails to operate satisfactorily. Under such a condition, constant current control doe not work, and thus the output current is gradually decreased as shown by a broken line in FIG. 2. For a region in which the output voltage departs from the normal charging range, especially for a region in which the output voltage goes below the lower limit voltage level as shown in FIG. 2, the property of the circuit is greatly influenced by scattering in the properties of the circuit elements. This has a remarkable effect on the properties of the circuit, and thus it turns out to be unstable so that the expected charging operation may fail to be performed. Another problem is that when the output voltage has gone below the lower limit voltage level, since the output current has been decreased. it takes a long time before the voltage recovers to the design range.
Such a problem could be solved by providing an independent reference constant current source which is free from the influence of the output voltage, but this approach is disadvantageous in that a complex and expensive circuit arrangement is needed to achieve this.